The biggest expansion of protection for the birds and sea life that live on the sandbanks, reefs and sea caves around the UK shores is announced today.

Fishing, dredging and other activities, including wind turbines, are to be banned or restricted on at least 15 new sites around the coast, ranging from Norfolk to Cornwall and Northern Ireland. They will protect species including sponges, sea squirts, corals, shellfish, and more exotic creatures such as the colourful cuckoo wrasse fish.

If all the UK's nominations are accepted by the European commission the total protected area would more than double to cover more than 4% of the national seas.

A spokesman for the Department for Environment, Food and Rural Affairs (Defra) said the estimated cost of the 15 sites was a one-off figure of nearly £4m followed by an annual extra budget of £1.3m for monitoring and enforcement by the new Marine Management Organisation.

The announcement was welcomed by conservation groups. The Wildlife Trusts said it was "a significant step towards the UK government realising its commitment to establishing an ecologically coherent network of marine protected areas".

Andrew Dodd, head of site conservation policy for the Royal Society for the Protection of Birds, said: "We are generally concerned, but like everybody else we will wait to see how government decides its priorities in the autumn. To date it hasn't been seen as a priority, we just hope this great announcement demonstrates the government's commitment to our wonderful marine life."

Defra will announce a list of new marine areas it wants to be included in the wider European Natura 2000 network.

Thirteen of them are nominated as special areas of conservation (SACs) for their important habitats, namely coral reefs, sandbanks and sea caves. The locations include waters off the coast of north Norfolk, much of the Devon coast, Britain's most southerly mainland point at the Lizard, and North-West Rockall Bank off the coast of Northern Ireland.

Another two sites – in Liverpool Bay and the Outer Thames Estuary – are being put forward as special protection areas, a designation reserved for birds, because they are important feeding grounds for the common scoter and red-throated diver .

The government is also opening consultations on another three possible areas, Lune Deep in Morecambe Bay, another section of the Devon coast, and Dogger Bank off north-east England.

If accepted – and officials said the decisions were expected to be rubber-stamped by the EU – the total marine protected areas would reach more than 200. However, the proportion of sea protected will still be less than half that on land.

Richard Benyon, the marine environment minister, said: "Our seas are home to some of the most diverse species and habitats in the world and they need just as much protection as our land."

A Defra spokesman denied that budget cuts would damage the ability to implement the new protected areas. But the estimated coast of the 15 new sites being put forward was a one-off cost of nearly £4m followed by an annual extra budget of £1.3m for monitoring and enforcement by the new , said the official.

"They won't have someone special going out there: this cost for enforcement will be part of their day to day business," he added. Conservationists have warned that the government risked a legal challenge if it did not meet obligations under European directives because of cuts.